74 lines
1.8 KiB
Go
74 lines
1.8 KiB
Go
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//go:build !amd64 || appengine || !gc || noasm
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// +build !amd64 appengine !gc noasm
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package zstd
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import (
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"errors"
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"fmt"
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)
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// buildDtable will build the decoding table.
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func (s *fseDecoder) buildDtable() error {
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tableSize := uint32(1 << s.actualTableLog)
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highThreshold := tableSize - 1
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symbolNext := s.stateTable[:256]
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// Init, lay down lowprob symbols
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{
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for i, v := range s.norm[:s.symbolLen] {
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if v == -1 {
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s.dt[highThreshold].setAddBits(uint8(i))
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highThreshold--
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v = 1
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}
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symbolNext[i] = uint16(v)
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}
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}
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// Spread symbols
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{
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tableMask := tableSize - 1
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step := tableStep(tableSize)
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position := uint32(0)
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for ss, v := range s.norm[:s.symbolLen] {
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for i := 0; i < int(v); i++ {
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s.dt[position].setAddBits(uint8(ss))
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for {
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// lowprob area
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position = (position + step) & tableMask
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if position <= highThreshold {
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break
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}
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}
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}
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}
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if position != 0 {
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// position must reach all cells once, otherwise normalizedCounter is incorrect
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return errors.New("corrupted input (position != 0)")
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}
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}
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// Build Decoding table
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{
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tableSize := uint16(1 << s.actualTableLog)
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for u, v := range s.dt[:tableSize] {
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symbol := v.addBits()
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nextState := symbolNext[symbol]
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symbolNext[symbol] = nextState + 1
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nBits := s.actualTableLog - byte(highBits(uint32(nextState)))
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s.dt[u&maxTableMask].setNBits(nBits)
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newState := (nextState << nBits) - tableSize
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if newState > tableSize {
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return fmt.Errorf("newState (%d) outside table size (%d)", newState, tableSize)
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}
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if newState == uint16(u) && nBits == 0 {
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// Seems weird that this is possible with nbits > 0.
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return fmt.Errorf("newState (%d) == oldState (%d) and no bits", newState, u)
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}
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s.dt[u&maxTableMask].setNewState(newState)
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}
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}
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return nil
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}
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